There is a security interest in detecting radioactive materials, such as special nuclear materials (“SNMs,” such as, for example, uranium (235U, 233U) and plutonium (239Pu)), at airports, seaports, border crossings, public places, and other locations. Conventionally, radiation detectors for identifying SNMs are effective only in close proximity to the SNMs because radiation intensity drops at a rate proportional to 1/r2, where r is the distance between the SNM and the radiation detector. SNMs may emit gamma rays and neutrons. Detection of SNMs is conventionally accomplished by detecting characteristic gamma rays, which may be substantially attenuated by shield materials. Neutrons from the SNMs (e.g., so-called “fast” neutrons) are difficult to detect with conventional detectors and are often not distinguished from background neutron emission (e.g., so-called “thermal” neutrons). Radiation detection methods are primarily reliant on ionization and scintillation processes, which often require high voltage biasing and/or electro-optical assemblies that complicate field deployment. Inorganic scintillators, for example, exhibit a nonlinear response and poor energy resolution, which limits their use for high resolution spectroscopic applications. Some conventional radiation detectors require the growth of crystals that are expensive and fragile. Furthermore, many conventional neutron detectors do not have the capability to provide imaging or directional information.
Turning to another technology, frequency selective surfaces (FSS) are used in a wide variety of applications including radomes, dichroic surfaces, circuit analog absorbers, and meanderline polarizers. An FSS is a two-dimensional periodic array of electromagnetic antenna elements. Such antenna elements may be in the form of, for example, conductive dipoles, loops, patches, slots or other antenna elements. FSS structures generally include a metallic grid of antenna elements formed on a dielectric substrate. Each of the antenna elements within the grid defines a receiving unit cell.
An electromagnetic wave incident on the FSS structure will pass through, be reflected by, or be absorbed by the FSS structure. This behavior of the FSS structure generally depends on the electromagnetic characteristics of the antenna elements, which can act as resonance elements. As a result, the FSS structure can be configured to function as low-pass, high-pass, or dichroic filters. Thus, the antenna elements may be designed with different geometries and different materials to generate different spectral responses.
The inventor has appreciated the need for improving upon existing technologies and to provide methods, structures, and systems associated with detecting radiation, such as radiation from radioactive materials (e.g., SNMs).